This invention relates to signal processing apparatus, and particularly to such apparatus as used in systems for simulating a graphic arts process. The apparatus of the invention is particularly useful in connection with a signal transforming apparatus such as a video signal generator which transforms light signals into electrical signals. In a graphic process, an original color image, such as a color transparency, is used in conjunction with filters to provide a set of color separations, which are black and white transparencies representative of a particular elemental color content in a composite color picture. In the usual graphic arts process, the color separations are used in the etching of color printing three or four different color inks on the same piece of paper using printing plates made from the color separations.
The various steps in the color printing process include adjustments for the regulation of the color content of the eventual print. These adjustments are usually made by a skilled operator who must make adjustments according to his judgement to come out with an acceptable color print. In many cases, the process must be repeated after printing plates have been made and a "proof" made with plates. The operator on viewing the proof again must use his own judgement to determine what adjustments to the process are required and must then go through the process of modifying the color separations or printing plates in order to obtain an acceptable color print.
In order to aid in the process of making the printing plates and to attempt to eliminate the need for multiple proof printing, there have been developed prior art systems for simulating the color printing process. Such prior systems are disclosed in U.S. Pat. Nos. 3,123,666 to Bailey: 3,128,333 to Loughlin; 3,131,253 Farber et al.; and 3,800,071 to Reeber, all of which are assigned to the same assignee as the present invention. These prior patents disclose previewing systems which simulate either all, or a portion of, the color printing process and provide a display representative of a color print. The present invention is particularly useful in systems of this type.
In a color printing process previewing system, it is particularly important that there be obtained a very accurate video signal which is representative of the original image. Unlike conventional color TV which used separate signals for brightness (i.e. black and white) and color information (with reduced bandwidth), previewing usually use at least three separate video signals, each representing the elemental color content of the original image. This results in a higher quality color image simulation and display. The three video signals may be derived by the use of a vidicon tube, according to the method taught by Farber et al., or may be derived by using a scanning light source and a photodetector tube. In either case, it is desirable to convert the video. signals into logarithmic signals in order to facilitate their subsequent processing. However, the resulting logarithmic signals may have a dynamic range which is less than that which can be optimally processed in subsequent circuits. In that case it would be desirable to be able to change the logarithmic scale of the signal so that a selected range of logarithmic representative voltages is fully utilized to represent the video signal.
It is therefore an object of the present invention to provide apparatus which is capable of changing the logarithmic scale of a supplied logarithmic signal.